The primary objective of these studies is to determine whether platelets (pits) can be stored for longer than the currently-licensed 5 days. Furthermore, does the duration of pit storage depend on the type of pit product being stored (apheresis pits, pit concentrates prepared from pit-rich plasma (PRP), or from buffy coats (BC), pre-storage pooled PRP pit concentrates, or pathogen-reduced apheresis or BC pits), and the medium used for storage;i.e., plasma or a storage solution. The four critical questions that will be ad- dressed in our studies are: 1) does the method of pit collection using apheresis procedures versus prepar- ing pit concentrates from whole blood influence storage duration?;2) does pre-storage pooling of pit con- centrates affect pit viability?;3) do pit storage solutions allow pits to be stored longer than pits stored in plasma?;and 4) can pathogen-reduced pits be stored for extended time periods similar to non-pathogen reduced pits? Although in vitro tests will be performed to document post-storage pit counts as well as a va- riety of assays to determine post-storage pit function, metabolism and apoptosis, the post-storage quality of the extended stored pits will be based on in vivo measurements in normal volunteers and thrombocytopenic patients. Specifically, radiolabeled pit recovery and survival measurements of extended stored autologous pits in normal volunteers will be used to determine post-storage pit viability for all types of pit products stored in plasma versus a storage solution. For pits that are stored for longer than 8 days and/or are stored in a storage solution, transfusion studies in the thrombocytopenic patients will be used to evaluate pit viabil- ity by measuring the radiolabeled recovery and survival of the donors'pits following transfusion. Alterna- tively, patient responses to transfused donor pits will be determined by measuring post-transfusion pit in- crements, corrected count increments, and days-to-next transfusion. Pit function (i.e., hemostasis) following the transfusion of extended stored donor pits will be monitored by pit count versus bleeding time measure- ments and by radiochromium-labeled stool blood loss studies. At the conclusion of these studies, we should know how long each type of pits can be stored in plasma or Plasmalyte, the relative merits of each type of pits, and whether the extended stored pits are both viable and functional when given to thrombocytopenic patients.